Field of the Invention
This invention relates to a multistage compressor unit comprising at least two different compressor elements driven by means of separate electric motors with an adjustable speed, whereby the outlet of a compressor element of one stage is connected to the inlet of a successive compressor element of a successive stage.
Contrary to the volume flow rate and the pressure ratio, the mass flow rate of such multistage compressor unit is constant in each of the stages.
Due to the different volume flow rate and the different pressure ratio, the speed of each compressor element is different and is determined by the output pressure and the final volume flow rate.
In some known two-stage compressor units with variable speed, the means for driving the compressor elements of the two stages comprise a single large electric standard motor which is driven by means of a large invertor or frequency regulator.
This motor drives the compressor elements by the intermediary of one large gearwheel.
The compressor elements have a built-in pressure ratio and belong to a series of elements which were designed such that they can be applied in one stage as well as in several stages, whereby then a minimum number of compressor elements reaches an entire range of air capacities.
Furthermore, the inertion of a larger motor with a large gearwheel is relatively high, as a result of which the response of the compressor unit is relatively slow, unless the motor is over-dimensioned.
As a result of the fixed speed ratio between the compressor elements of the different stages, the efficiency of the compressor unit is restricted over its complete working range. The present compressor units have only one optimum efficiency for one well-defined output pressure and volume flow rate.
From JP 07158576 A in the name of Kobe Steel Ltd, a two-stage compressor unit is known, the two compressor elements of which are driven by separate motors, whereby the speed of the motors is adjusted by means of an invertor. In an embodiment, the two invertors are controlled by means of a same control device in function of the pressure between the two stages. In another form of embodiment, the invertors are controlled by separate control devices, in function of the pressure between the stages, the pressure at the exit of the high-pressure stage, respectively.
The compressor element of the low-pressure stage is larger than the compressor element of the high-pressure stage, and the nominal rotational speeds of the compressor elements are different. Therefore, the compressor element of the high-pressure stage is driven without transmission by means of a smaller motor than the compressor element of the low-pressure stage which is driven by means of a gear transmission and by a larger motor. This construction is relatively complicated and expensive.
JP 02140477 A also describes a two-stage compressor unit, in which two similar compressor elements are installed in one housing and are driven directly by motors, the speed of which is regulated separately by an invertor. The efficiency of such compressor unit, however, is not optimum.
The invention aims at a multistage compressor unit which does not show the aforementioned disadvantages, is relatively economic and can work in a simple manner with an optimum efficiency.
According to the invention, this aim is achieved in that in the compressor unit, as defined in the first paragraph, the electric motors are identical and therefore have an approximately identical nominal capacity, whereas between each motor and the compressor element driven thereby, a gear transmission is provided.
In spite of the fact that the compressor elements are different in order be able to operate in an optimum manner, the motors, however, are identical. Therefore, motors of the same type and with the same nominal capacity which already are on the market can be used, which allows to reduce the price.
If the compressor unit comprises two stages and, therefore, two compressor elements, hereby the one gear transmission, in particular the one at the low-pressure stage, may cause a speed reduction in respect to the rotational speed of the corresponding motor, whereas the other gear transmission, namely, the one at the high-pressure stage, causes a speed increase in respect of the rotational speed of the corresponding motor.
By an efficient selection of the motors, both gear transmissions, as well as the motors, can be identical, whereby both gear transmissions comprise a large and a small gearwheel which are exchanged in the one gear transmission in respect to the other gear transmission.
These motors preferably are high-speed motors.
Preferably, the electric motors are coupled to their own frequency regulator, such that the frequency and, therefore, the speed can be regulated separately per motor.
The invention also relates to a method for regulating a multistage compressor unit according to any of the preceding forms of embodiment, which therefore comprises a identical electric motor per compressor element which is fed by means of a pertaining frequency regulator, such that the frequency and, therefore, the speed can be regulated separately per motor, wherein the speed ratio between the motors of the different stages is adjusted continuously in order to obtain an optimum overall efficiency.
Energy saving is achieved by adjusting the speed ratio of the stages and, therefore, the pressure ratio between the different stages in such a manner that, apart from a desired output pressure, an optimum overall efficiency of the compressor unit is obtained.
The optimum efficiency of the compressor unit is obtained by optimizing the speed of each stage and, therefore, the pressure ratio over each stage.
During this adjustment of the speed ratio, the output pressure is measured and, in function thereof, the speed of one of the motors is adapted immediately. This motor, mostly called xe2x80x9cmasterxe2x80x9d, either may be the motor of the low-pressure stage or the motor of the high-pressure stage.
The optimum speed and, therefore, pressure ratio on each stage is known and present in a databank or can be calculated by means of an algorithm, for example, a fuzzy control, in real time.
After altering the speed of this motor, the optimum speed ratio is determined by means of a databank or an algorithm in function of the speed of said motor and the measured output pressure in order to thereby adapt the speed of the other motors.
Preferably, the speed ratio between the motors is determined for each condition of the compressor unit in function of the measured output pressure and is taken from a databank or is calculated by means of a real-time algorithm.
With the intention of better showing the characteristics of the invention, hereafter, as an example without any limitative character, a preferred form of embodiment of a multistage compressor unit and of a method for regulating such multistage compressor unit according to the invention is described, with reference to the accompanying drawing which schematically represents such compressor unit.